JP2005-110452 discloses a switching power supply circuit having a primary circuit connected to a primary coil of a transformer and a secondary circuit connected to a secondary coil of a transformer, wherein an electrode pattern for the primary circuit and an electrode pattern for the secondary circuit are disposed in such a manner that they are opposed to each other. An insulating layer between the electrode patterns functions as a dielectric for a capacitor and thus forms an equivalent capacitor. The equivalent capacitor functions as a capacitor for noise reduction.
A non-isolated DC-DC converter, for example, as shown in FIG. 1, is provided with a first loop circuit and a second loop circuit. The first and second loop circuits share an inductance L and have capacitors C1 and C2, respectively. A voltage conversion is implemented by turning on and off switching element Q1 or Q2 provided in the first or second loop circuit. During the switching, the first and second capacitors have functions of smoothing an output of the DC-DC converter and reducing a noise level generated by the DC-DC converter. Generally, such a circuit configuration as shown in FIG. 1 is implemented by providing the first and second loop circuits on a common surface or separate surfaces of a printed circuit board, as shown in FIG. 2.
However, in the conventional circuit configuration as shown in FIG. 1 and FIG. 2, when the first switching element Q1, for example, is operated to turn ON/OFF, the current passes through the first and second loop circuits alternately, and thus a magnetic field through the first loop circuit and a magnetic field through the second loop circuit are generated alternately. At that time, a direction of the current passing through the first loop circuit and a direction of the current passing through the second loop circuit are opposite to each other, as shown by arrows in FIG. 1, and thus a direction of the magnetic field through the first loop circuit and a direction of the magnetic field through the second loop circuit are opposite to each other. In such a configuration, magnetic fields in opposite directions are generated alternately for a short period due to a high-speed (short term) ON/OFF operation of the switching element Q1, resulting in a problem that a noise is generated due to such a variation in the magnetic field.